Tropical Storm Olga began to weaken slightly Wednesday as it entered the waters between Cuba and Hispaniola, where heavy rain forced the evacuation of low-lying areas.

The rare December cyclone had maximum sustained winds near 45 mph, down from 50 mph earlier in the day. Forecasters warned its heavy rain could trigger dangerous floods by dumping as much as 10 inches of rain on Hispaniola, the island shared by the Dominican Republic and Haiti.

Olga struck nearly two weeks after the official end of the Atlantic hurricane season. It is only the 10th named storm to develop in the month of December since record keeping began in 1851, according to the U.S. National Hurricane Center in Miami.

“It’s not completely unusual to have a storm form in December,” said Daniel Brown, a hurricane specialist at the center, who noted that three named storms have formed after Nov. 30 since 2003.

At 7 a.m. EST, Olga was centered about 110 miles west-northwest of Port-au-Prince, Haiti’s capital, and about 65 miles off the eastern tip of Cuba, according to the hurricane center.

The storm passed through the southwestern areas of the Dominican Republic that were hardest hit by Tropical Storm Noel six weeks ago. At least 87 fatalities in the country were blamed on Noel, the deadliest storm of the 2007 Atlantic hurricane season.

Authorities evacuated 22 communities in eastern provinces to prepare for the storm.

“Every civil defense agency has been activated,” emergency services spokesman Luis Luna Paulino said.

In neighboring Haiti, the government earlier had warned people in coastal areas to prepare for evacuations and urged fishermen to remain in port, said Marie Alta Jean-Baptiste, head of the country’s civil protection department.

Olga was moving westward at about 19 mph. Forecasters predicted it would gradually weaken into a tropical depression as its center continues across the Caribbean between Cuba and Jamaica.

The storm passed Puerto Rico on Tuesday night, knocking out electrical service to 79,000 people and water to 144,000.

A tropical storm warning was in effect for the northern coast of Hispaniola, the southeastern Bahamas and the Turks and Caicos Islands, meaning tropical storm conditions were expected there within 24 hours. Tropical storm conditions were also possible over parts of eastern Cuba.

Olga will be included in the tally for the 2007 hurricane season, bringing the number of named storms to 15, including six hurricanes. The next season begins June 1.